Photoacoustic Spectroscopy (PAS) provides information about the optical and thermal properties of condensed phase materials. A theoretical model is presented of photoacoustic processes in both the frequency and the time domain. The model includes instantaneous deexcitations, the relaxation time of a sample with a two-level optical absorption band, and a two-layer absorbing system as three cases of practical interest. Expressions are developed for the dependence of the photoacoustic cell pressure upon the optical absorption coefficient and the excited state lifetime, and the modifications introduced by the microphone transfer function are discussed. It is shown that in both measurement modes thermally thick and optically opaque specimens are preferred. In the Frequency Domain emphasis is placed on the effect of material parameters on the phase of the PAS response. Circumstances under which the phase may be used to measure the optical absorption coefficient of the specimen and the excited state lifetimes are defined. The value of the phase measurement in the study of surface films is discussed. In the Time Domain the PAS response curves are interpreted in terms of physical processes occurring in the cell. The limitations of the technique for excited state lifetime measurements and surface film detection are discussed and the experimental conditions necessary for optimum relaxation time and surface film thickness determinations are evaluated. The model presented suggests that the PAS technique is suitable for the measurement of lifetimes greater than ca. 10⁻⁵ sec⁻³ and detection of overlayer thicknesses greater than ca. 10⁻³ cms, with the limits arising from the microphone transfer function, the finite sound velocity, and the thermal transport velocity in the transducer gas.